The production and availability of nitrogen as a product has been a desired goal which has been achieved with varying degrees of success in the past. The use of such nitrogen product has generally been on a small scale on a volumetric basis.
Recently, the use of nitrogen in large quantities has found utility in the maintenance and enhancement of petroleum recovery operations. Previously, such petroleum reserves, after depletion of natural pressure, were either terminated or natural gas co-recovered with the petroleum was reintroduced as a pressurizing medium for the petroleum. As the cost of both petroleum and natural gas have risen, it has become desirable to recover petroleum in low pressure or non-naturally producing reservoirs, and it has also become desirable to use pressure maintaining or pressure enhancing mediums other than natural gas.
In order to make such alternate pressurizing media cost effective, large quantities of the medium must be available at very low cost. Industries have turned to nitrogen as a readily available source of an inert pressurizing medium which is available in large quantities throughout the world. Large air separation plants have been constructed to provide the necessary quantities of nitrogen for pressure maintenance or enhanced petroleum recovery. In order to maintain nitrogen as an attractive medium for petroleum recovery operations, the cost must be maintained as low as possible. Various attempts to produce large quantities of nitrogen under efficient circumstances so as to have a cost effective quantity of nitrogen have been attempted by those skilled in the art.
In British Pat. No. 1,215,377 an air separation apparatus is set forth wherein nitrogen is produced as product of the air separation. Air is initially compressed and cooled before being cleansed of water and carbon dioxide in switching adsorbent beds. A portion of the cleaned air is then expanded through a work producing expansion means before the entire air stream is introduced into the high pressure stage of a two stage, low and high pressure distillation column. The overhead and the bottom stream from the high pressure column are introduced into the low pressure column as reflux to the low pressure column, respectively. A reboiler-condenser connects the low pressure column and the high pressure column thermodynamically. A portion of the nitrogen recovered in the condenser of the high pressure column is removed as product and rewarmed. A portion of the oxygen enriched waste from the bottom of the low pressure column is removed and expanded in order to condense nitrogen in the overhead of the low pressure column, while the enriched oxygen waste is reboiled and removed as a waste stream. A second nitrogen product at low pressure is removed from the upper region of the low pressure column as a product and is rewarmed, along with the other process streams from the column. However, this patented cycle delivers all of its feed air to the high pressure column and does not deliver any feed air directly to the low pressure column. This reduces the potential efficiency of the separation system. This system must also compress the feed air to a relatively high pressure, because the entire feed air stream is expanded to a reduced pressure, which is still equal to the pressure of the high pressure stage of the distillation column. This also would result in decreased efficiency.
Another two stage distillation column system for the generation of nitrogen product is set forth in U.S. Pat. No. 4,222,756 wherein the feed air is delivered entirely to a high pressure stage of the distillation column and refrigeration is supplied in large part by expansion of the entire nitrogen overhead from the high pressure stage through a turbine with delivery of the expanded nitrogen to the mid-section of the low pressure stage of the distillation column. A portion of the nitrogen is removed as product from the top of the low pressure stage of the distillation column while the remainder condenses in a vaporizer-condenser driven by oxygen enriched waste from the base of the low pressure stage in the distillation column. Various alternate nitrogen producing air separation plants are set forth in this patent in FIG. 1, FIG. 2 and FIG. 3. None of these cycles provide large quantities of nitrogen at the efficiency of operation of the present invention.
The present invention overcomes the drawbacks in efficiency of the prior art for the production of large volumes of nitrogen by providing a system which provides only the required high pressure column feed to generate the optimum low pressure column boilup vapor from the reboiler-condenser. The remaining portion of the total air feed is fed directly to the low pressure column. By minimizing the portion of the total feed air compressed to feed the high pressure column, the total energy input is minimized.
In addition, by uncoupling the expander flow from mass balance considerations, only the required air flow is taken to the expander. This reduces inefficiency in the heat exchanger-expander system by reducing requirements for bypasses around the expansion system.